There are many vital biological processes, such as membrane fusion and fat digestion, which must involve, if only locally and transiently, changes in lipid phase state. To date, however, little attention has been paid to the dynamics of lip phase interconversions. If, indeed, such changes are physiologically relevant, they must occur on a time-scale comparable with hose taking place in vivo. Thus, there is a need to establish the kinetics of lipid phase transitions. Such information is integral to our understanding of the structural and compositional requirements of transitions that occur in biological, model and reconstituted systems. A method, referred to as time-resolved x-ray diffraction (TRXRD), has been developed which facilitates the direct and quantitative measurement of bulk lipid phase transitions as they occur in live time. Insights into mechanism are provided by the ability to detect transition intermediates and by a detailed analysis of the static and time-resolved x-ray diffraction data collected during the transformation. Kinetic and mechanism studies are performed by using TRXRD in conjunction with relaxation measurements following the imposition of a rapid perturbation in one or more thermodynamic variables. This study utilizes TRXRD to unravel the structural and dynamic interrelationships of lipid mesomorphs, and to establish the time-scale upon which phase transitions take place and the possible occurrence of transition intermediates. Mechanism refinement is performed by analyzing diffraction data recorded during the transition and by establishing the dependence of rate and mechanism on molecular structure, composition and environmental factors such as temperature, pressure, and electric and magnetic field strength. The ultimate goal of this study is to understand lipid mesomorphism, the mechanism of lipid phase transitions and the structural relatedness of the different mesomorphs. This will augment our appreciation of the physiological role of lipids and lipid diversity and our understanding of the physicochemical properties of these liquid crystalline materials in intact biological and reconstituted membranes and other lipid-rich aggregrates. The proposed measurements rely critically on the availability of high brilliance synchrotron x-radiation and will involve the development and application of new technologies, experimental strategies and data processing procedures.